The present invention relates to an ink jet recording head for forming an image on a recording medium by ejecting ink droplets on the recording medium. The present invention also relates to a method of manufacturing the ink jet recording head.
An ink jet recording head, which employs, for example, an piezoelectric elements as a pressure applying system, has a passage unit. The passage unit is formed by arranging a passage forming plate and an elastic plate in a layered manner on a nozzle plate having a plurality of nozzle orifices, and then the passage unit is fixed to or bonded to a case. The passage forming plate is formed with partition walls which define pressure generating chambers to be communicated with the respective nozzle orifices, a common ink chamber for storing ink to be supplied to the pressure generating chambers, ink supplying portions communicating the common ink chamber with the pressure generating chambers, and so on. The nozzle plate is bonded to one of the surfaces of the passage forming plate, while an elastic plate is bonded to the other surface of the same, thereby forming the passage unit. Piezoelectric elements are fixedly disposed on the case side so as to correspond in position to the respective pressure generating chambers. By attaching the passage unit to the case, each of the piezoelectric elements is brought into contact with and fixed to a corresponding part of the elastic plate where a corresponding one of the pressure generating chambers is disposed. The head thus constructed serves in the following manner: Ink is supplied from the common ink chamber to the pressure generating chambers. Each piezoelectric element, when driven, presses the elastic plate to increase pressure inside the corresponding pressure generating chamber, so that ink droplet is ejected from the corresponding nozzle orifice using the increased pressure.
The elastic plate is typically in the form of a two-layered structure made up of a high polymer film of 2 to 10 .mu.m, such as a PPS (polyphenylene sulfite) film or a polyimide film, and a stainless steel plate of several ten microns. By subjecting the blank of the two-layered structure to photo etching process, a portion of the stainless steel plate, which corresponds in location to the common ink chamber is removed, while portions of the stainless steel plate, corresponding in location to the piezoelectric elements remains. The remaining portions of the stainless steel plate serves as thick portions with which the respective piezoelectric elements are fixedly contacted. The reason why the portion of the stainless plate corresponding in position to the common ink chamber is removed to expose the high polymer film is to prevent cross talk possibly occurring between adjacent nozzles as well as to eliminate print density variation.
Increase of pressure within a pressure generating chamber by pressurizing action of a piezoelectric element causes not only an ejection of an ink droplet from a corresponding nozzle orifice, but also an ink jet flow (i.e. a reverse flow) from the pressure generating chamber through the ink supplying portion to the common ink chamber. Consequently, pressure variation occurs within the common ink chamber to influence internal pressure of an adjacent pressure generating chamber, thereby varying ejection characteristic of an adjacent nozzle. In order to avoid such cross talk, a high polymer film serving as an elastic film is used to seal the common ink chamber (that is, the common ink chamber is at least partially defined by the elastic high polymer film) to increase compliance of the common ink chamber. This arrangement can absorb a pressure variation occurring within the common ink chamber due to the above-mentioned ink jet flow.
When the number of ejecting nozzles and the ejection frequency abruptly vary, a flow rate of the ink also abruptly varies. Therefore, a pressure wave is generated within the ink supplying portion by the water hammer action. This pressure wave also propagates from the common ink chamber to the pressure generating chambers via the ink supplying portion to affect the ejection characteristics of the nozzles. That is, variation of print density occurs depending on frequency cycle of the pressure wave. Therefore, in order to stabilize the print density, the high polymer film is used to seal the common ink chamber to increase the compliance of the common ink chamber, thereby absorbing a pressure variation within the common ink chamber due to the pressure wave, and coping with the variation of the ink flow rate.
It is, however, found out that in practice the ink jet recording head needs more compliance of the common ink chamber to completely prevent the cross talk and the print density variation.
The high polymer film of PPS or the like, rather than a metal plate, is adopted as the elastic film because the high polymer material can make the elastic film thinner in easier fashion and provide more compliance. However, a linear expansion coefficient of the high polymer film is larger than that of the silicon wafer constituting the passage forming plate or the stainless plate forming the thick portion of the elastic plate. Therefore, a tension acts on the high polymer film if temperature drops to room temperature after the postbaking is carried out to follow the dry film development during the photo etching process or after the passage forming plate is bonded to the elastic plate at high temperature. Further, most of high polymer films have such a nature that the high polymer films do not return to the original size once the high polymer films shrink at a predetermined temperature or higher. It is estimated that those characteristics or natures of the high polymer film causes a tension acting and remaining on the high polymer film after the high polymer film is bonded, which decreases the compliance to cause the cross talk and the print density variation. For this reason, it is considered that to secure a predetermined compliance, it is necessary to increase the size of the common ink chamber.
The size increase of the common ink chamber leads to increase of the head size. This is problematic in increasing the recording speed and reducing the entire size of a recording apparatus. Particularly in the case of the head having a multiple of nozzle arrays, which correspond to colors for color print, the size of the whole head depends largely on the width of the common ink chamber. In this respect, the size increase of the common ink chamber is not preferable.
More specifically, the manufacturing process for the passage unit, which includes a bonding step, includes a step or steps requiring temperature of 100.degree. C. or higher. In this case, the high polymer film shrinks about 1%.
The experiment and analysis, conducted by the inventors, showed the following facts. To prevent the print density variation, it is necessary that a maximum tolerable pressure within the common ink chamber is 2000 Pa or lower. The evaluation was conducted using a compliance of the common ink chamber at 2000 Pa as the reference. At 2000 pa, a compliance obtained in an assumed condition where no 1% shrinkage of the high polymer film is caused is about 16 times as large as a compliance value obtained when the 1% shrinkage is caused. Conversely, to secure a 16-fold compliance in a state that the high polymer film is shrunk by 1%, the width of the common ink chamber must be increased approximately 2.5 times since the compliance is approximately proportional to the third power or cube of the width of the common ink chamber.